1. Field of the Invention
This invention pertains to the field of diagnostic x-ray imaging, and more specifically to x-ray beam hardening filters.
2. Background
X-ray sources used in medical imaging are typically polychromatic, that is, the x-ray source produces x-ray photons with varying energies. For example, an x-ray source capable of producing a 120 keV photon will typically produce an x-ray beam having a mean energy of only one-third to one-half of the peak energy. Given that the mean energy is roughly one-half to one-third of the peak energy, many of the photons that comprise an x-ray beam will be characterized by energy levels below the mean energy.
A problem with lower energy photons is that they do not contribute to the construction of the radiographic image. Many of the lower energy photons, for example those with energies less than 20 keV, may be absorbed in the object under investigation; these lower energy photons only contribute to harmful patient radiation. Therefore, it is desirable to filter the lower energy x-ray photons from the x-ray beam.
It is known to use filters to remove lower energy photons from the x-ray beam. One form of filtration is inherent filtration. Inherent filtration results from the absorption of x-ray photons as they pass through the x-ray tube and its housing. Such filtration varies with the composition, or lining of the x-ray tube and housing, as well as the length of the x-ray tube and housing. Inherent filtration, which is measured in aluminum equivalents, typically varies between 0.5 and 1.0 mm aluminum equivalent.
A second form of filtration is added filtration. Added filtration is achieved by placing an x-ray attenuator or filter in the path of the x-ray beam. Most materials have the property of attenuating the lower energy photons more strongly than higher energy photons. When lower energy x-ray beams strike the added filter they are absorbed. By adding a filter to the x-ray beam path, lower energy x-ray photons can be absorbed, thereby reducing the unnecessary radiation created by the lower energy x-ray photons. Because the lower energy x-ray photons are preferentially removed from the x-ray beam, the mean energy of the x-ray beam is increased. Increasing the mean energy of the x-ray beam is referred to as "hardening" of the x-ray beam.
Objects to be x-rayed vary in thickness and composition. Thus, it is desirable to control the amount of filtration that occurs. Some x-ray systems, having a relatively small diameter x-ray source, often use a filter consisting of a thin sheet of aluminum or aluminum and copper. The filter is placed in the path of the x-ray beam, either manually or by an electromechanical actuator. Because of the small diameter of the x-ray source, the filter and filter control mechanism can be made compact.
However, when a large-area x-ray source (e.g., having a diameter of approximately 25 cm or larger) is used in an x-ray imaging system and if added filtration is used, the beam hardening filter inserted into the path of the x-ray beam would be as large as the overall x-ray source in order to cover the entire source. Furthermore, the mechanical travel of the filter to insert it into the path of the x-ray beam would also be about the same as the size of the x-ray source (e.g., 25 cm) or the filter. Using a conventional x-ray hardening filter, for example one that slides in a parallel plane to the surface of the x-ray source, on a large-area x-ray source would involve a large mechanical actuator assembly and would add undesirable bulk to the x-ray imaging system.